Temperature compensation for QKD systems

FIELD OF THE INVENTION

0001The present invention relates to quantum cryptography, and in particular relates to systems and methods for temperature compensation for a quantum key distribution (QKD) system.

BACKGROUND OF THE INVENTION

0002Quantum key distribution involves establishing a key between a sender (“Alice”) and a receiver (“Bob”) by using weak (e.g., 0.1 photon on average) optical signals transmitted over a “quantum channel.” The security of the key distribution is based on the quantum mechanical principle that any measurement of a quantum system in unknown state will modify its state. As a consequence, an eavesdropper (“Eve”) that attempts to intercept or otherwise measure the quantum signal will introduce errors into the transmitted signals, thereby revealing her presence.

0003The general principles of quantum cryptography were first set forth by Bennett and Brassard in their article “Quantum Cryptography: Public key distribution and coin tossing,” Proceedings of the International Conference on Computers, Systems and Signal Processing, Bangalore, India, 1984, pp. 175–179 (IEEE, New York, 1984). Specific QKD systems are described in U.S. Pat. No. 5,307,410 to Bennett, and in the publication by C. H. Bennett entitled “Quantum Cryptography Using Any Two Non-Orthogonal States”, Phys. Rev. Lett. 68 3121 (1992). The general process for performing QKD is described in the book by Bouwmeester et al., “The Physics of Quantum Information,” Springer-Verlag 2001, in Section 2.3, pages 27–33.

0004The above-mentioned references describe a so-called “one-way” QKD system wherein Alice randomly encodes the polarization or phase of single photons, and Bob randomly measures the polarization or phase of the photons. The one-way system described in the Bennett 1992 paper and incorporated by reference herein is based on a shared interferometric system. Respective parts of the interferometric system are accessible by Alice and Bob so that each can control the phase of the interferometer. The signals (pulses) sent from Alice to Bob are time-multiplexed and follow different paths. As a consequence, the interferometers need to be actively stabilized to within a few tens of nanoseconds during transmission to compensate for thermal drifts.

0005U.S. Pat. No. 6,438,234 to Gisin (the '234 patent), which patent is incorporated herein by reference, discloses a so-called “two-way” QKD system that is autocompensated for polarization and thermal variations. Thus, the two-way QKD system of the '234 patent is less susceptible to environmental effects than a one-way system.

0006When operating a QKD system in practice (e.g., in a commercial setting), multiple variables need to be aligned in time and then maintained aligned for optimal system performance. For example, in a commercial QKD system one or more single-photon detectors (SPDs) are gated with one or more corresponding detector gating signals from a controller to synchronize the detection of optical pulses with expected pulse arrival times. However, once the system is set up, the timing drifts due to various systemic and environmental factors (e.g., temperature) and the photon count can drop. This leads to a reduction in the transmission rate of the system, and also to an increase in the bit—error rate—i.e., to diminished system performance.

0007Laboratory and prototype QKD systems can be adjusted to account for system drifts can under very controlled and artificial conditions. However, making the same kinds of adjustments for a commercial QKD system in the field is a far more daunting endeavor. And, unlike with a laboratory or prototype QKD system, end-users of commercial QKD systems have an expectation that their QKD system will automatically run in an optimal state with minimal or no operator intervention.

BRIEF DESCRIPTION OF THE DRAWINGS

0008<figref idref="DRAWINGS">FIG. 1</figref> is a schematic diagram of a two-way QKD system;

0009<figref idref="DRAWINGS">FIG. 2A</figref> is a close-up schematic diagram of the controller for the QKD system of <figref idref="DRAWINGS">FIG. 1</figref>, illustrating the elements that provide thermal compensation of the detector gating signal for the SPD;

0010<figref idref="DRAWINGS">FIG. 2B</figref> is a close-up schematic diagram of the SPD control board of Bob of <figref idref="DRAWINGS">FIG. 2A</figref>;

0011<figref idref="DRAWINGS">FIG. 3A</figref> is a plot of the SPD photon count versus the timing of the detector gating signal illustrating the optimum detector gating signal timing t_MAXas indicated by the maximum photon count number N_MAX;

0012<figref idref="DRAWINGS">FIG. 3B</figref> is a timing diagram of detector gating signals as a function of temperature;

0013<figref idref="DRAWINGS">FIG. 4</figref> is a schematic diagram of a one-way QKD system; and

0014<figref idref="DRAWINGS">FIG. 5</figref> is a close-up schematic diagram of the controller for the QKD system of <figref idref="DRAWINGS">FIG. 4</figref>, illustrating the elements that provide thermal compensation of the detector gating signal for the two SPDs in the SPD unit.

0015The various elements depicted in the drawings are merely representational and are not necessarily drawn to scale. Certain sections thereof may be exaggerated, while others may be minimized. The drawings are intended to illustrate various embodiments of the invention that can be understood and appropriately carried out by those of ordinary skill in the art.

SUMMARY OF THE INVENTION

0016A first aspect of the invention is a method of providing temperature compensation for the timing of a gating signal for a single-photon detector (SPD) in a QKD station of a QKD system. The method includes determining a reference detector gating signal timing value corresponding to an ambient reference temperature of the QKD station, e.g., at or near an SPD electronics control board. The method also includes incrementally varying and measuring a temperature of the QKD station over a range of temperature values, and determining a change in timing from the reference detector gating signal timing value for each measured temperature value. The method further includes storing the measured temperature value and the corresponding change in timing of the detector gating signal in a look-up table, operating the QKD station at an operating temperature that varies over time within the range of temperature values, and then adjusting the timing of an operational detector gating signal by an amount associated with the operating temperature as defined in the look-up table.

0017A second aspect of the invention is a method of providing temperature compensation for the timing of a gating signal for a single-photon detector (SPD) in a QKD station of a QKD system. The method includes incrementally varying and measuring the temperature of the QKD station over a range of temperature values, determining an optimum detector gating signal timing value for each measured temperature value, and storing the measured temperature value and the corresponding optimal detector gating signal timing value in a look-up table. The method further includes operating the QKD station at an operating temperature that varies over time within the range of temperature values, and adjusting the timing of the detector gating signal to correspond to that associated with the corresponding operating temperature based on the temperature and timing values stored in the look-up table.

0018In general, the relationship between the timing drift and the temperature may be a simple equation. In this case, a lookup table is not required. For example, it might be determined that the relation ship is linear, such as 100 ps of timing drift per degree C. In such a case, the method includes adjusting the operating temperature based on the simple equation.

0019A third aspect of the invention is a method relating to the above-described aspects, and further including providing an uncompensated detector gating signal from a timing/synchronization unit that controls the timing and synchronization of the QKD system to a summation unit, providing the look-up table timing value to the summation unit in response to a temperature signal representative of the operating temperature, forming in the summation unit a compensated detector gating signal, and then providing the compensated detector gating signal to the SPD.

DETAILED DESCRIPTION OF THE INVENTION

0020The present invention relates to systems and methods of thermally compensating one or more timing signals gating one or more SPDs in a QKD system in order to maintain optimal system performance. In particular, the present invention involves compensating for errors in detector gating due to thermal drifts to maintain the optimal detector gating signal position (timing) during the QKD system's operation. This results in optimal optical pulse detection in the QKD system, which generally corresponds to the optimal operation of the QKD system as a whole. In the discussion below, the various signals S<b>1</b>, S<b>2</b>, S<b>3</b>, etc., can be considered digital voltage signals.

0021The invention is applicable to one-way, two-way, free-space and ring topology, n-way QKD systems, etc., using either polarization encoding or phase encoding, and using one or more SPDs. The invention is first described below in connection with an example embodiment of a two-way QKD system using phase-encoding and a single SPD located in one of the QKD stations (Bob). The invention is also described in connection with a one-way phase-encoding system having two SPDs. These choices are merely for the sake of illustration and are not intended as limiting the invention to these types of QKD systems.

0022Also, in the description below, a “gating signal” is a signal that activates the element to which the signal is sent, wherein the activation of the element corresponds to the duration (width W) of the signal. Thus, the detector gating signal activates the SPD for the duration (i.e., width) of the detector gating signal, wherein activation starts at the leading edge of the signal and ends at the trailing edge of the signal.

0000Two-Way QKD System Embodiment

0023<figref idref="DRAWINGS">FIG. 1</figref> is a schematic diagram of an example embodiment of a two-way QKD system <b>200</b> to which the methods of the present invention are aptly suited. System <b>200</b> includes two key encoding stations: a transmitting/receiving station Bob and a reflecting station Alice, referred to hereinafter simply as “Bob” and “Alice.”

0000Bob

0024With continuing reference to <figref idref="DRAWINGS">FIG. 1</figref>, Bob includes a laser <b>202</b> that emits initial light pulses <b>204</b>. Laser <b>202</b> is coupled to a time-multiplexing/demultiplexing optical system <b>206</b> having an input end <b>208</b>A, an input/output end <b>208</b>B, and a detector output end <b>208</b>C. Optical system <b>206</b> receives input pulses <b>204</b> at input end <b>208</b>A, splits each pulse into two time-multiplexed pulses P<b>1</b> and P<b>2</b> and outputs them at input/output end <b>208</b>B. Likewise, optical system <b>206</b> also receives optical pulses at input/output end <b>208</b>B, as described below.

0025A single-photon detector (SPD) unit <b>216</b> having one or more SPDs is coupled to optical system <b>206</b> at detector output end <b>208</b>C. In the present example, SPD unit has two SPDs <b>216</b>A and <b>216</b>B coupled to output end <b>208</b>C via respective optical fiber links F<b>1</b> and F<b>2</b> (<figref idref="DRAWINGS">FIG. 2</figref>). A phase modulator (PM) <b>220</b> is coupled (e.g., by an optical fiber) to optical system input/output end <b>208</b>B. An optical fiber <b>240</b> connects Bob to Alice at PM <b>220</b>.

0026Bob also includes a controller <b>248</b> operatively (e.g., electrically) coupled to laser <b>202</b>, SPD unit <b>216</b>, and PM <b>220</b> to control the operation of these elements, as described below.

0027<figref idref="DRAWINGS">FIG. 2A</figref> includes a more detailed view of controller <b>248</b>. Controller <b>248</b> includes a timing/synchronization unit <b>250</b> coupled to laser <b>202</b>, phase modulator <b>220</b> and to a corresponding timing/synchronization unit <b>289</b> in Alice's controller <b>288</b> (see <figref idref="DRAWINGS">FIG. 1</figref> and the description for Alice below). Controller <b>248</b> coordinates the generation of optical pulses <b>204</b> and the modulation and detection of optical pulses P<b>1</b> and P<b>2</b> (<figref idref="DRAWINGS">FIG. 1</figref>) exchanged between Alice and Bob. An example timing/synchronization system is disclosed in pending PCT patent application serial no. PCT/US2004/03299, entitled “QKD systems with robust timing,” which patent application is incorporated herein by reference and which is published as PCT Publication No. WO 2004/0732354 A2.

0028With reference to <figref idref="DRAWINGS">FIG. 2B</figref>, timing/synchronization unit <b>250</b> and SPD unit <b>216</b> are included as part of a SPD control board <b>249</b>, wherein the timing/synchronization unit generates a SPD gating signal S<b>3</b> and delivers it to the SPD unit, as described in greater detail below.

0000Bob's Controller

0029With reference again to <figref idref="DRAWINGS">FIG. 2A</figref>, controller <b>248</b> further includes a processor <b>251</b> (e.g., a microprocessor) coupled to a memory device <b>252</b> (e.g., a ROM, PROM, EPROM, EEPROM, etc.) and optionally coupled to timing/synchronization unit <b>250</b> via line <b>257</b>. Processor <b>251</b> is available to re-program memory device <b>252</b> and to perform other processing functions, as described below. Memory device <b>252</b> is adapted to store data in a register or look-up table. Memory device <b>252</b> is connected to a summation unit <b>253</b> also included in the controller. Memory device <b>252</b> is also connected to a temperature sensor TS (not shown in <figref idref="DRAWINGS">FIG. 1</figref>). In an example embodiment, temperature sensor TS is located at or near the SPD control electronics board <b>249</b> (see <figref idref="DRAWINGS">FIG. 2B</figref>) so as to measure the ambient temperature thereat or thereabout.

0030In an example embodiment, temperature sensor TS is a direct-to-digital sensor. In another example embodiment, temperature sensor TS provides an analog temperature signal (not shown) and is connected to an analog-to-digital converter ADC, which in turn is connected to memory device <b>252</b>. ADC converts the analog temperature signal to a digital temperature signal ST for storage in the (digital) memory device <b>252</b>.

0031The summation unit <b>253</b> is adapted to receive data in the form of a timing adjustment signal SS from timing/synchronization unit <b>250</b> and data in the form of a signal SM from memory device <b>252</b>. Summation unit <b>253</b> is further adapted to combine the data (signals) SS and SM to form a compensated detector gating timing signal S<b>3</b>′, as described below. Summation unit <b>253</b> is coupled to SPD unit <b>216</b>.

0032In an example embodiment, processor <b>251</b>, memory device <b>252</b> and temperature sensor TS are included in the controller as a single module, such as the DS1847 Dual Temperature-Controlled NV Variable Resistor (digital Potentiometer) module available from Maxim Integrated Products, Sunnyvale, Calif.

0033In an example embodiment, temperature sensor TS (and optionally analog-to-digital converter ADC used with an analog temperature sensor) is coupled to processor <b>251</b> via processor line (link) PL. Processor <b>251</b> is then available to process temperature signals ST directly prior to the temperature information being stored in memory device <b>252</b>, as opposed to storing the unprocessed temperature information directly into the memory device.

0034In one example embodiment, processor <b>251</b> receives temperature information via signal ST from temperature sensor TS, and also receives timing information directly from timing/synchronization unit <b>250</b> via signal SS sent over line <b>257</b>. The processor then calculates a functional relationship between the detector gating signal timing and the temperature, and provides calculated temperature vs. detector gating timing information to memory unit <b>252</b> via a signal SL, or directly to timing adjustment unit <b>253</b> via a processor signal SP sent over (dashed) line <b>258</b>.

0000Alice

0035With reference again to <figref idref="DRAWINGS">FIG. 1</figref>, Alice includes a variable optical attenuator (VOA) <b>264</b> connected to optical fiber <b>240</b>. A phase modulator (PM) <b>266</b> is arranged downstream of and is optically coupled to VOA <b>264</b>. A Faraday mirror <b>270</b> is arranged downstream of and is optically coupled to PM <b>266</b>.

0036Alice also includes a controller <b>288</b> operatively (e.g., electrically) coupled to PM <b>266</b> and VOA <b>264</b>. Controller <b>288</b> also includes a timing/synchronization unit <b>289</b> coupled to PM <b>266</b> and to Bob's timing/synchronization unit <b>250</b> (<figref idref="DRAWINGS">FIG. 2A</figref>). Controllers <b>248</b> and <b>288</b> are linked (e.g., electrically or optically) via link <b>290</b> to synchronize the operation of Alice and Bob through the coordinated operation of the respective timing/synchronization units <b>289</b> and <b>250</b>. In particular, the operation of the phase modulators <b>220</b> and <b>266</b>, and SPD unit <b>216</b> are controlled and coordinated by controllers <b>248</b> and <b>288</b> relative to the timed launch of optical pulses <b>204</b> using gating signals S<b>2</b>, S<b>1</b> and S<b>3</b>, respectively, when exchanging a quantum key between Alice and Bob. Thus, controllers <b>248</b> and <b>288</b> can in a sense be considered as constituting a single controller for the QKD system.

0000Idealized QKD System Operation

0037With reference to <figref idref="DRAWINGS">FIGS. 1 and 2A</figref>, in the idealized operation of system <b>200</b>, an optical pulse <b>204</b> is generated by laser source <b>202</b> in response to a timing signal S<b>0</b> from timing/synchronization unit <b>250</b>. Optical pulse <b>204</b> is divided into two separate pulses P<b>1</b> and P<b>2</b> by time-multiplexing/demultiplexing optical system <b>206</b>. In the present example embodiment, pulses P<b>1</b> and P<b>2</b> are relatively weak pulses The pulses are passed out of optical system input/output end <b>208</b>B to PM <b>220</b>, which is gated “off” to allow the pulses to pass therethrough unmodulated. Pulses P<b>1</b> and P<b>2</b> then pass to Alice over optical fiber <b>240</b>. Pulses P<b>1</b> and P<b>2</b> continue to VOA <b>264</b>, which can attenuate the pulses if necessary. The pulses then pass through PM <b>266</b> and are reflected by Faraday mirror <b>270</b>, and then pass back through PM <b>266</b> a second time.

0038During one of the passes of pulses P<b>1</b> and P<b>2</b> through PM <b>266</b>, the PM modulates one of the pulses—say, pulse P<b>1</b>—to form a phase-modulated pulse P<b>1</b>′. This is achieved by controller <b>288</b> sending a well-timed gating signal S<b>1</b> that activates PM <b>266</b> for the short period of time (i.e., less than the time-separation between the pulses) when pulse P<b>1</b> passes through PM <b>266</b>. Pulses P<b>1</b> and P<b>2</b> then pass back through VOA <b>264</b>, which can attenuate the pulses if necessary. The pulses then pass back to Bob and to PM <b>220</b> therein. PM <b>220</b> is then directed to randomly modulate one of the returning pulses—say unmodulated pulse P<b>2</b>—with one of the select phase modulation values. This is achieved by controller <b>248</b> providing a well-time gating signal S<b>2</b> to PM <b>220</b> that activates the phase modulator during the short time period within which pulse P<b>2</b> passes through PM <b>220</b>.

0039Now-modulated pulses P<b>1</b>′ and P<b>2</b>′ (the latter is not shown in <figref idref="DRAWINGS">FIG. 1</figref>) continue on to optical system <b>206</b>. Optical system <b>206</b> combines the pulses to form a combined pulse P<b>3</b>, which is directed out of detector output end <b>208</b>C to SPD <b>216</b> unit and into fiber F<b>1</b> or F<b>2</b>, depending on the overall phase of combined pulse P<b>3</b>. SPD unit <b>216</b> receives a detector gating signal S<b>3</b> timed to coincide with the arrival of combined pulse P<b>3</b>. One of detectors <b>216</b>A and <b>216</b>B in SPD unit <b>216</b> receives pulse P<b>3</b>, depending on the overall phase imparted to pulses P<b>1</b> and P<b>2</b>, and outputs a signal to controller <b>248</b> indicating the arrival of a photon in the corresponding SPD.

0040Once a desired number of optical pulses are exchanged, the key is derived using known techniques—for example, by Alice and Bob publicly comparing the basis of their measurements and only keeping the measurements (bits) corresponding to the same measurement basis. This forms the sifted key. They then choose a subset of the remaining bits to test for the presence of an eavesdropper Eve and then discard these bits. The act of eavesdropping on optical fiber <b>240</b> by Eve intercepting or otherwise attempting to measure the weak optical pulses being transmitted between Bob and Alice will necessarily introduce errors in the key due to the quantum nature of the photons being exchanged. If there are no errors in the sifted key due to the presence of an eavesdropper Eve, then the transmission is considered secure, and the quantum key is established.

0000Thermal Compensation Operation

0041The above description of QKD system <b>200</b> is for idealized operation. In practice, however, thermal effects within Bob can cause a change in the timing (e.g., jitter) of the operational detector gating signal S<b>3</b>. Accordingly, in an example embodiment, the effect of temperature on operational detector gating signal S<b>3</b> is quantified. This is accomplished in an example embodiment by incrementally changing (e.g., ramping up) the temperature of Bob over a select temperature range RT (e.g., 0 to 50° C. in 0.2° C. increments) and recording (e.g., via temperature sensor TS) each temperature value in memory device <b>252</b>. The temperature ramping may be achieved, for example, by placing Bob in an oven having sufficient temperature-control sensitivity.

0042Also, for each temperature value in the temperature range as recorded by temperature sensor TS, the QKD system is operated and the timing of the detector gating signal S<b>3</b> is adjusted.

0043In one example embodiment, the adjustment is based on the timing of the detector gating signal S<b>3</b> that provides an optimal number of photon counts in SPD unit <b>216</b>. <figref idref="DRAWINGS">FIG. 3A</figref> is an example plot of the results of a detector gating signal scan, wherein the Y-axis is the number N of photon counts obtained during the detector gating interval (i.e., the gate pulse width W). The X-axis represents the relative timing (e.g., arrival time t) of the detector gating signal S<b>3</b>, which is varied for each temperature value to achieve the maximum number of photon counts N_MAXassociated with an optimum detector gating signal timing position t_MAX.

0044Data points d<b>1</b>-d<b>4</b> are shown on the plot. In the context of the present invention, the maximum number of photon counts N_MAXcorresponds to optimal system performance because it corresponds to the highest data transmission rates and highest photon sensitivity level vs. timing, with no increase in dark current counts. The curve in <figref idref="DRAWINGS">FIG. 3A</figref> is obtained by incrementing the arrival time t of detector gating signal S<b>3</b> over a first select range R<b>1</b> of timing values t (X-axis) to find a first maximum photon count, and then over a second select range R<b>2</b> (R<b>2</b>&lt;R<b>1</b>) to find the local maximum photon count. In an example embodiment, the arrival time t corresponds to the position of the leading edge of the detector gating signal relative to a reference, e.g., a clock reference time provided by timing/synchronization unit <b>251</b>.

0045In another example embodiment, the position of the detector gating signal S<b>3</b> is measured using an oscilloscope and the timing difference from a reference timing position (value) is recorded. The timing difference is also recorded in memory device <b>252</b>.

0046Also as discussed above, the temperature vs. detector gating signal timing is calculated by processor <b>251</b> based on detector gating signal timing information in signal SS from timing/synchronization unit <b>250</b> and temperature information from signal ST from temperature sensor TS and provided to memory device <b>252</b> via signal SL (or alternatively directly to summation unit <b>253</b> via processor signal SP).

0047In each case, the result is a temperature T vs. detector gating signal timing offset Δ (“T vs. Δ”) look-up table for the select range of temperatures. The look-up table provides information relating to proper detector gating timing for a given temperature value.

0048<figref idref="DRAWINGS">FIG. 3B</figref> is a timing diagram illustrating the variation Δ in detector gating signal timing with temperature T that forms the look-table set forth below in Table 1.

0049<tables id="TABLE-US-00001" num="00001"><table frame="none" colsep="0" rowsep="0"><tgroup align="left" colsep="0" rowsep="0" cols="1"><colspec colname="1" colwidth="217pt" align="center" /><thead><row><entry namest="1" nameend="1" rowsep="1">TABLE 1</entry></row></thead><tbody valign="top"><row><entry namest="1" nameend="1" align="center" rowsep="1" /></row><row><entry>T vs. Δ</entry></row></tbody></tgroup><tgroup align="left" colsep="0" rowsep="0" cols="3"><colspec colname="offset" colwidth="63pt" align="left" /><colspec colname="1" colwidth="77pt" align="left" /><colspec colname="2" colwidth="77pt" align="left" /><tbody valign="top"><row><entry /><entry>T</entry><entry>Δ</entry></row><row><entry /><entry namest="offset" nameend="2" align="center" rowsep="1" /></row><row><entry /><entry>TR</entry><entry>0</entry></row><row><entry /><entry>T1</entry><entry>Δ2</entry></row><row><entry /><entry>T2</entry><entry>Δ3</entry></row><row><entry /><entry>T3</entry><entry>Δ4</entry></row><row><entry /><entry>Tn</entry><entry>Δn</entry></row><row><entry /><entry namest="offset" nameend="2" align="center" rowsep="1" /></row></tbody></tgroup></table></tables>

0050In an example embodiment, a reference detector gating signal S<b>3</b>R corresponding to a reference temperature TR is established to provide a baseline reference for optimum gating signal timing.

0051Thus, in operating QKD system <b>200</b> to compensate for thermal variations in detector gating signal timing, temperature sensor TS provides ongoing temperature measurements via temperature signal ST to memory device <b>252</b>. In an example embodiment, processor <b>251</b> periodically obtains a temperature measurement via temperature signal TS and calculates any change in temperature from previous temperature readings, e.g., as stored in memory device <b>252</b>.

0052If a change in temperature has occurred beyond some threshold temperature increment (e.g., 0.2° C.), then memory device <b>252</b> sends a timing adjustment signal SM to summation unit <b>253</b>. Timing adjustment signal SM carries timing information about detector gating signal S<b>3</b> (e.g., the proper timing position or the required timing delay) from the T vs. S<b>3</b> look-up table for the temperature measured at temperature sensor TS. If no change in temperature has occurred, or if the change is below the threshold temperature change, then no signal is sent, or alternatively adjustment signal SM indicates zero adjustment.

0053Further, timing/synchronization unit <b>250</b> sends a timing/synchronization signal SS to summation unit <b>253</b> that includes the uncompensated detector gating signal timing information (i.e., the uncompensated detector gating signal timing position). Summation unit <b>253</b> receives signals SM and SS and forms therefrom a time-compensated detector gating signal S<b>3</b>′. The compensated detector gating signal S<b>3</b>′ includes a timing adjustment added to the operational detector gating signal S<b>3</b> to properly compensate for the change in timing due to the thermal variation at Bob per the look-up table. In other words, in an example embodiment, the detector gating signal is compensated to restore the operating detector gating signal timing to that of the reference signal.

0054In an example embodiment, the timing of compensated detector gating signal S<b>3</b>′ is that which yields an optimum number of photon counts at SPD unit <b>216</b>. In an example embodiment, the compensated detector gating signal S<b>3</b>′ is the reference detector gating timing signal SR in <figref idref="DRAWINGS">FIG. 3B</figref>.

0055In another example embodiment, the timing of the compensated detector gating signal is set by summation unit <b>253</b> in response to a function calculated in processor <b>251</b> based on data for the temperature T vs. detector gating signal timing adjustment Δ from the look-up table or provided directly to the processor during the data-taking phase.

0000One-Way QKD System Embodiment

0056<figref idref="DRAWINGS">FIG. 4</figref> is a simplified schematic diagram of an example one-way QKD system <b>400</b>. The same elements from prior Figures are given the same reference numbers for the sake of illustration. System <b>400</b> includes at Alice laser <b>202</b>, phase modulator <b>266</b> arranged in the optical path of the laser, and controller <b>288</b> operably coupled to laser <b>402</b> and modulator <b>266</b>. Also included is an optical modem <b>408</b>A optically downstream of phase modulator <b>266</b> that links (multiplexes) sync channel <b>290</b>A with optical fiber (quantum channel) <b>240</b>. Bob includes an optical modem <b>408</b>B coupled to optical fiber <b>240</b> and Bob's sync channel <b>290</b>B. Bob also include a modulator <b>206</b> downstream of optical modem <b>408</b>B, and two detectors <b>216</b>A and <b>216</b>B arranged downstream of the phase modulator. The two detectors are coupled to phase modulator <b>206</b> via a 50—50 beamsplitting coupler <b>420</b>.

0000Idealized QKD System Operation

0057In conventional operation, controller <b>288</b> provides control signal S<b>0</b> to laser <b>202</b>, which in response thereto emits an initial optical pulse <b>204</b>. Pulse <b>204</b> travels through phase modulator <b>266</b>, which randomly modulates pulse <b>206</b> according to a particular QKD protocol, via a timed gating signal S<b>2</b> from controller <b>288</b>. The modulated pulse <b>204</b>′ proceeds to optical modem <b>408</b>A and is coupled into optical fiber <b>240</b>. Also, sync signal SS generated by controller <b>288</b> travels over sync channel <b>290</b>A to optical modem <b>408</b>A and is coupled into optical fiber <b>240</b>.

0058Optical pulse <b>204</b>′ and sync signal SS proceeds over to Bob, where they are received by optical modem <b>408</b>B. Sync signal SS is directed to sync channel <b>290</b>B and into controller <b>248</b>, while optical pulse <b>204</b>′ proceeds to phase modulator <b>206</b>. The latter modulates the phase of optical pulse <b>204</b>′ via a timed gating signal S<b>1</b> from controller <b>248</b>, thereby forming a twice-modulated optical pulse <b>204</b>″ (<figref idref="DRAWINGS">FIG. 5</figref>). The modulation is performed according to the QKD protocol being used for the system. Optical pulse <b>204</b>″ then proceeds to coupler <b>420</b>, which directs the optical pulse to one of detectors <b>216</b>A and <b>216</b>B, depending on the overall modulation of the optical pulse. The pulses are detected by controller <b>248</b> sending timed gating signals S<b>3</b>A and S<b>3</b>B to SPDs <b>216</b>A and <b>216</b>B, respectively.

0000Thermal Compensation for One-Way System

0059<figref idref="DRAWINGS">FIG. 5</figref> is a detailed schematic diagram of controller <b>248</b> of system <b>400</b>. Controller <b>248</b> of system <b>400</b> includes the same elements as described above in connection with system <b>200</b> of <figref idref="DRAWINGS">FIG. 2A</figref>. Accordingly, the operation of controller <b>248</b> in providing thermal compensation for QKD system <b>400</b> is essentially the same as described above in connection with QKD system <b>200</b>. In QKD system <b>400</b>, temperature sensor TS is arranged at or near the two SPDs <b>216</b>A and <b>216</b>B control board. Further, lines LA and LB connecting SPDS <b>216</b>A and <b>216</b>B to summation unit <b>253</b> are made the same length so that compensated detector gating signals S<b>3</b>′ arrive at the SPDs at the correct time. Alternatively, one of the compensated detector gating signals S<b>3</b>′ is delayed to account for any path difference between lines LA and LB.

0060In the foregoing Detailed Description, various features are grouped together in various example embodiments for ease of understanding. The many features and advantages of the present invention are apparent from the detailed specification, and, thus, it is intended by the appended claims to cover all such features and advantages of the described apparatus that follow the true spirit and scope of the invention. Furthermore, since numerous modifications and changes will readily occur to those of skill in the art, it is not desired to limit the invention to the exact construction, operation and example embodiments described herein. Accordingly, other embodiments are within the scope of the appended claims.